1. Field of the Invention
This invention relates to a semiconductor acceleration detecting device and, more particularly, to a semiconductor acceleration detecting device mounted to a moving body, such as a motor vehicle, for detecting an acceleration applied thereto.
2. Description of the Related Art
FIG. 9 illustrates a known semiconductor acceleration detecting device. As shown FIG. 9, in the known semiconductor acceleration detecting device, a semiconductor chip 1 is fixed to a pedestal or a mounting bed 5 in a cantilevered manner at an end 1b thereof. Thus, the other end 1c of the semiconductor chip 1 acts as a free end or edge. The semiconductor chip 1 is made, for example, of a semiconductor material such as silicon and is flexible. Moreover, the semiconductor chip 1 is fixed by means of the pedestal 5 to a base 6 which is to be attached to an object (or a moving body) to be measured so that it is used to detect acceleration of the object. As illustrated in FIG. 9, the semiconductor chip 1 consists of a weight portion 2 provided at the one end 1c thereof serving as a free end, a thin wall portion 3 and a strain gauge 4 provided in the thin wall portion 3. The strain gauge 4 is connected with an external electrical circuit (not shown) through a wire 7. Here, note that the thin wall portion 3 is formed by reducing the thickness of a part of the semiconductor chip 1, as illustrated in this figure, so as to improve the sensitivity of the strain gauge 4 for detecting strain occurring in the semiconductor chip 1.
Hereinafter, an operation of the known semiconductor acceleration detecting device will be described. When an acceleration a is applied to the object, the acceleration a is also applied to the semiconductor chip 1 through the base 6 and the pedestal 5. Then, if M denotes the mass of the weight potion 2, a force F=ma is generated in the weight portion 2. This weight portion 2 is formed in such a manner as to be integral with the thin wall portion 3 and is further supported by the thin wall portion 3. Thus, a stress proportional to the force F is generated in the thin wall portion 3. The resistance of the strain gauge 4 changes according to this stress. Therefore, the acceleration a applied to the object is detected by converting the change in the resistance of the strain gauge 4 into an electrical signal.
As above described, in the known semiconductor acceleration detecting device, the acceleration a applied to the object is transmitted to the semiconductor chip 1 through the base 6 and the pedestal 5. Therefore, the known semiconductor acceleration detecting device has problems. When the connection or adhesion between the base 6 and the pedestal 5 or between the pedestal 5 and the semiconductor chip 1 is incomplete, the acceleration a is not accurately transmitted to the semiconductor chip 1 and an erroneous acceleration may be measured without noticing the abnormality of such an acceleration.
When the pedestal 5 is completely detached from the base 6 or the semiconductor chip 1, the acceleration a cannot be detected at all, though the acceleration a is actually applied to the object.